Pretensioners are used in modern vehicle seat belt systems to rapidly draw in a length of belt webbing in the event of a crash and take up slack in the belt. This pretensioning action minimizes forward movement of the restrained vehicle occupant and more correctly positions the vehicle occupant in the vehicle seat to maximize the effect of a secondary safety restraint, such as an airbag.
Vehicle manufacturers prefer pretensioners to be as small as possible and this has led, in recent years, to the development of various forms of so-called compact pretensioners. In general a compact pretensioner works without the need for a clutch mechanism to connect it to the retractor.
In addition, modern seat belt systems often incorporate load limiting features to reduce the detrimental effect of the force exerted on the vehicle occupant by the restraining seat belt itself. It is desirable to lock the seat belt and pretension it at the very beginning of a crash, as the crash force is rising, but to then allow a small amount of payout of the seat belt, at a particular controlled load at the peak of the crash force, to reduce the peak load felt by the vehicle occupant.